The invention relates to ceramic moulds for metal casting. More particularly, the invention relates to a ceramic mould for precision metal casting, a slurry used to fabricate a ceramic mould, and a process for producing a ceramic mould.
Ceramic moulding or ceramic mould casting is a foundry process aimed at economically providing a high degree of precision and outstanding metal soundness in the production of cast parts with no presently known size restriction. Conventional ceramic mould casting processes yield the type of tolerances equivalent to lost wax precision casting and at a significantly lower cost, comparable to sand casting processes, for casting a small number of parts. However, ceramic moulding processes still require improvements, and there is a constant demand for increased accuracy, improved surface finish, increased consistency of castings produced, and for simplification of the process. Examples of conventional ceramic mould casting processes can be found in U.S. Pat. Nos. 2,795,022; 2,811,760; 2,931,081; 3,022,555; and 3,172,176.
Currently, the most commonly used ceramic mould casting process is the Shaw process or its variants, such as the Unicast process. In these processes, a ceramic mould is first fabricated by admixing a binder, a gelling agent and comminuted refractory material (reduced to minute particles by crushing, grinding, or pulverising) to form a slurry. The slurry is then cast around a pattern and is allowed to set, after which time the pattern is stripped from the set ceramic mould and the mould is stabilised.
The binder used in the slurry typically comprises a lower alkyl silicate, such as ethyl silicate, which yields an alcohol on hydrolysis and which is sufficiently volatile to burn when ignited. The refractory material is selected so that it can withstand the high heat and does not react with the molten metal during casting. The refractory material is normally composed of two or more grades of ingredients having different particle size: the finer grade of ingredients imparts a smooth surface finish to the casting and the coarser grade of ingredients, in appropriate proportions, reduces the shrinkage and distortions in the mould.
The stabilisation treatment xe2x80x9cfixesxe2x80x9d the dimensions of the ceramic mould after which it will not change during the subsequent baking and casting process. In the Shaw process, after setting and stripping the pattern, the mould is immediately subjected to a rapid, uniform and intense flame firing, whereby all of the volatiles escape from the set mould. The rapid burning and intense heat cause micro-cracks to develop (known as xe2x80x9ccrazingxe2x80x9d), which renders a dimensional freezing, so that the mould is immune to subsequent severe thermal shocks.
In the Unicast process, after stripping the pattern from the set mould, the mould is not immediately ignited by a flame but is either immersed in or sprayed with a hardening liquid to chemically stop excessive gelling reaction of the binder, and thus the mould dimensions are stabilised. The hardening liquid is miscible with alcohol. The mould can then be ignited to burn off most of the volatiles before it is fired at elevated temperatures. The Unicast process does not require immediate burning of alcohol from the ceramic mould and hence simplifies the operation of the mould-making process.
An inherent problem in the conventional ceramic mould fabrication processes is that the mould is usually subject to distortions, such as twisting, warpage and cracking.
These distortions deteriorate the accuracy of the mould and increase surface irregularities. Sometimes cracking results in complete destruction of the mould. This problem originates from shrinking of the mould while the binder is gelling.
U.S. Pat. No. 3,690,366 discloses a process that reduces the excessive shrinkage of the mould by decreasing the amount of binder necessary to render the slurry flowable. However, decreasing the amount of binder in the slurry requires an increase in the amount of coarse particles in the refractory mixture, making it difficult to achieve a smooth surface finish on the resulting metal casting. To obtain a smooth surface finish, high proportions of fine particle ingredients are required, calling for the use of more binder. Thus, a compromise between obtaining a reasonable surface finish and reducing mould distortion and cracking must be reached. In practice, it is not a trivial task to determine the optimal refractory composition for a particular application. In addition, because the actual size, distribution and shape of particles vary with different producers and with different batches, it becomes very difficult to maintain a consistent surface finish and dimensional accuracy among the castings produced.
Other conventional ceramic mould casting processes include the fabrication of composite ceramic moulds to reduce mould distortions (such as warpage and twisting) and mould cracking, and to reduce costs by using inexpensive materials for the backing layer. In one of the processes for making composite ceramic moulds, crushed ceramic mould fragments made in accordance with the Shaw process are incorporated into the ceramic slurry for making the new ceramic mould. The main purpose of this process is to reduce the considerable distortions, such as warpage and twisting, associated with the original Shaw process.
However, the use of composite ceramic moulds has still not proven satisfactory either for the production of metal castings requiring very tight tolerances or for applications where those tight tolerances need to be consistent among different moulds made from the same pattern. In practice, the slurry does not adhere well to the crushings or slabs or to the backing layer as it gels. Such a mould is weak and unstable and the heat of the molten metal causes the mould to break apart.
Another process for making ceramic composite moulds produces a mould having a two-layer structure: a facing layer and a backing layer. An example of such a process can be found in U.S. Pat. No. 5,368,086. The facing layer is made of materials with suitable refractoriness for the casting process, usually higher than that of the backing layer, and contains more fine refractory ingredients to produce smooth surface finishes for the castings. There are two different practical variations of this process. One variation makes the facing layer first and then, after setting of the facing layer, makes the backing layer. The other variation reverses the order, making the backing layer first over an oversized pattern and then, after setting of the backing layer, making the facing layer by pouring a slurry in the gap formed between the backing layer and a pattern having the final dimensions. After the two layers have set, the composite body of the mould is ignited to remove volatiles and is further fired at elevated temperatures before casting.
In addition to the advantage of reducing cost, an apparent benefit of these two-layered moulds is that the properties of the backing layer and the facing layer can be adjusted independently to achieve optimum results. For example, the backing layer may be made from coarser particles to allow the volatiles to escape readily. At the same time, a highly refractory and very fine facing layer could be formed to resist the heat of the molten metal and to provide a smooth casting surface.
A disadvantage of two-layered moulds is that the gelled slurry expands when it is subsequently fired or baked, but the backing layer does not expand by the same amount. As a result, separations can occur between the surfaces of the hardened slurry and the surfaces of the backing layer. In the cases where xe2x80x9cinexpensivexe2x80x9d backing materials are used, the refractoriness of the backing layer is usually much lower than that of the facing layer, leading to more distortions and dimensional inaccuracies in the casting produced using such moulds. Thus, achieving the same tolerance among two moulds made from the same pattern is extremely difficult and, as a practical matter, can only be attained randomly.
Other problems are also inherent in two-layer mould fabrication processes. For example, there is no reliable way to judge the optimal gelling time of the facing layer. The optimal gelling time depends on a variety of factors, such as average particle size, volume of refractory material, gelling agent/accelerator, water, and mixing time, all of which tend to differ with each slurry prepared. If the facing layer is not permitted to gel for a long enough period of time, it will run or deform under the influence of its own gravitational forces after the two-layered mould is removed from the pattern, causing changes in the shape of the mould and thus affecting accuracy. On the other hand, if the facing layer is permitted to gel for too long a period of time, it will not adhere well to the backing layer. These problems are encountered even when the facing layer is formed first. A further disadvantage is that the two-layered mould process is more complex, involves more steps, and is more time consuming than a single layer process.
As mentioned above, the dimensional stability of a ceramic mould during the casting process relies on the stabilisation treatment. However, a considerable amount of the distortion (e.g. warpage and twisting) and cracking occurs before and during the stabilisation treatment process. An important factor affecting the distortion and cracking of the ceramic mould is the amount of liquid binder used in the slurry, which depends on the composition of the slurry. Generally, the more binder in the slurry, the more distortion and cracking occur in the mould.
A method of reducing dimensional changes and distortions before and during the stabilisation treatment process is disclosed in the U.S. Pat. No. 3,690,366 which suggests that the ratio of binder, in ml, to 100 g refractory should be kept below 33.6/xcfx81, where xcfx81 is the density of the packed refractory powder. This document states that a slurry meeting this requirement will avoid mould cracking during stabilisation treatment by drying in the atmosphere.
There is a need for a process for ceramic mould casting that allows for a reduced amount of binder in the slurry without detrimentally increasing surface roughness. Further, there is a need for a simplified process that allows for formation of a high quality mould surface having high mould accuracy.
It is an object of the present invention to provide a ceramic mould, a slurry composition, and a process for forming a mould, which obviate or mitigate at least one disadvantage of the prior art.
In a first aspect, the present invention provides a ceramic mould slurry comprising a binder, a gelling agent, a first refractory material having a density of xcfx811 and a mean particle size of xcex12, and a second refractory material having a density of xcfx812 and a mean particle size of xcex12, wherein xcfx811 greater than 2 and xcex11 less than xcex12. Thus, the denser particles have a smaller mean particle size.
The first refractory material has a smaller mean particle size (xcex11) than that of the second refractory material (xcex12). This allows for the formation of a better mould and casting surface finish. The first refractory material can be of any acceptable size as determined by a person skilled in the art. For example, mean particle size of the first refractory material (xcex11) may be from about xe2x88x92100 to about xe2x88x92400 mesh, and mean particle size of the second refractory material (xcex12) may be coarser than about 100 mesh, for example, from about 20 to about 100 mesh.
The invention additionally provides a process for producing a ceramic mould comprising the steps of: (a) preparing a slurry comprising a binder, a gelling agent, a first refractory material having a density of xcfx811 and a mean particle size of xcex11, and a second refractory material having a density of xcfx812 and a mean particle size of xcex12, wherein xcfx811 greater than 2 and xcex11 less than xcex12; and (b) casting a ceramic mould using the slurry.
According to one embodiment of the invention, there is provided a slurry for preparing a ceramic mould for use in the ceramic mould casting process. The slurry contains at least two refractory materials with different densities. The materials differ in particle size, so that the denser material has a smaller mean particle size. Advantageously, the invention allows independent selection of fractions of fine and coarse refractory materials. Moulds produced according to the invention, and castings produced with such moulds have both a consistently high accuracy and a good surface finish. A further advantage of the instant invention over prior art two-layer processes is that the mould fabrication process is simplified. Further, the inventive process minimises the amount of binder in the slurry, while using coarse particles and still maintaining good surface finish.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.